September 04, 2008

Genomic ancestry of Mexicans

Individuals in an admixed population inherit a fraction of their genomic ancestry from the contributing ancestral groups, with individual chromosomal segments inherited from each of them (as e.g. 23andme's ancestry painting shows)

In this paper, researchers discovered that in particular regions of the genome, Mexicans, who are a 3-way mixture of Europeans, Amerindians, and Sub-Saharan Africans, tend to have lower than average African ancestry, or higher-than-average European ancestry.

This indicates that these regions have been under selection, which has managed, in the few centuries or so since the arrival of Europeans and Africans in the New World to (dis)favor particular chunks of DNA.

From the paper:

Admixture in the present day Mexican American population has occurred over the past 500 years, during the post-Columbian era of the New World. As noted before (FBPP Investigators 2002), this Latino population is composed primarily of European and Native American ancestry, with a modest amount of African ancestry. Our overall estimates of average Native American (39%), European (57%) and African (4%) ancestry in this population from Starr County, Texas are quite similar to what was observed for a sample of Mexicans from Mexico City by Wang et al. (2008), with corresponding proportions of 40, 57 and 3%, respectively.

...

Our results are different from those presented by Tang et al. (2007) in a study of ancestral admixture in Puerto Ricans. Those authors found evidence for excess African ancestry on chromosome 6p and excess Native American ancestry on chromosomes 8q and 11q. Despite both being Latino populations, Mexicans and Puerto Ricans are actually quite distinct regarding their genetic as well a social and demographic histories. On average, Mexicans are primarily European and Native American in ancestry, with a modest African contribution (Tang et al. 2006); by contrast, Puerto Ricans have substantial African ancestry and more modest Native American ancestry (Tang et al. 2007). It is intriguing to consider the possibility that different environmental exposures operated on these populations with distinct genetic admixtures to produce differing patterns of selection. Both populations were subjected to prevalent endemic infectious diseases in the years following the Spanish conquest, a time when selection may have operated in the formation of these new admixed populations.

Human Genetics doi: 10.1007/s00439-008-0541-5

Genome-wide distribution of ancestry in Mexican Americans

Analabha Basu et al

Abstract Migrations to the new world brought together individuals from Europe, Africa and the Americans. Inter-mating between these migrant and indigenous populations led to the subsequent formation of new admixed populations, such as African and Latino Americans. These unprecedented events brought together genomes that had evolved independently on different continents for tens of thousands of years and presented new environmental challenges for the indigenous and migrant populations, as well as their offspring. These circumstances provided novel opportunities for natural selection to occur that could be reflected in deviations at specific locations from the genome-wide ancestry distribution. Here we present an analysis examining European, Native American and African ancestry based on 284 microsatellite markers in a study of Mexican Americans from the Family Blood Pressure Program. We identified two genomic regions where there was a significant decrement in African ancestry (at 2p25.1, p < 10−8 and 9p24.1, p < 2 × 10−5) and one region with a significant increase in European ancestry (at 1p33, p < 2 × 10−5). These locations may harbor genes that have been subjected to natural selection after the ancestral mixing giving rise to Mexicans.

21 comments:

" As can be seen, the African ancestry is generally low, with a mean of 0.042, and also very modest variance among loci. On the other hand, European ancestry and Native American ancestry are much greater on average (mean of 0.57 and 0.39, respectively) and with much greater variance. "

The post says that those stats are almost the same as obtained for Mexico City (which is also in North America, btw) and it's highlighted.

This may be the same as the apportion mentioned by Sijia Wang et al, 2008 (doi:10.1371/journal.pgen.1000037) for a sample from Mexico City specifically described as "mestizo" (though one may note that that's the most common racial identity in Mexico).

If you do samples along the Mexican Southern border or near Cancun there will be much more black ancestry than 4%. Still, I've been to Mexico City, Guadalajara, and NYC. I also have Mexican American (and National) friends and associates as well as Puerto Rican and Dominican.

It is pretty obvious by appearance that on average Mexicans are far more Amerindian than Puerto Ricans (and especially Dominicans) who usually appear Mulatto although there is is a significant white Iberian phenotype in both populations; I would say this is more true of Mexicans than PRs. I have only seen pictures of "white Dominicans"...everyone I have met in person looks like a "light skin or dark skin black" as we would say in the U.S.

The few blacks that existed in Mexico were absorbed in the 19th century, there are still a handful of people with predominately West African phenotypes in some areas of Southern Mexico but many of those are mixed with Amerindian groups today.

The place that seemed to be the most segregated in the region was Cuba. Cuba has a large relatively unmixed black African and White Iberian population...which is unlike the other Hispanic Carribean islands. Not sure the history of that.

I spend 4 months these years, in Mazatlan, Mexico. I have gotten to know, and appreciate, the native indian culture: Huichol, Cora, etc.. It may not be common knowledge, but the mexicans are quite in favor of "segregation". The rich all tend to be descendants of Spanish/Caucasian heritage. Mazatlan, early in the 20th century expelled all asians from their city. I have never met a Jewish person from Mazatlan, nor Arabic (semitic) for that matter. Re: dark-skinned people, again I sense prejudice, they are called the Spanish word for Buckwheat? I would suggest that the culturalization of areas like Mazatlan was prejudicial and people who didn't fit in, probably left??? It is my sense now that the more caucasian blood you have, the better your chances of advancing in mexican society are. Note: gringos are tolerated, but generally aren't encouraged/permitted to become mexican citizens - that is apparently quite hard to do?

That's a blank statement and, sincerely, I doubt it to be true for most Mexicans.

This doesn't mean that there is not historical and even ongoing racism in Mexico, as well as in other parts of Latin America.

The rich all tend to be descendants of Spanish/Caucasian heritage.

That's probably true. But does not justify your previous claim. It rather accounts for historical discrimination dating back to colonial times. Racial/ethnic issues have been growing all through Latin America, sometimes violently, often mixed with class war, and nowadays we are witnessing dramatic changes or attempts of change of this historical racist status quo in most countries. Mexico is no exception, just that the change has not succeeded yet.

Nevertheless, it must be recalled that the most celebrated Mexican President, Benito Juárez, was a native american.

Mazatlan, early in the 20th century expelled all asians from their city. I have never met a Jewish person from Mazatlan, nor Arabic (semitic) for that matter.

The Chinese community had problems in the late 19th and early 20th century largely because they leant in favor of the USA and against the Mexican Revolution. Or so I have read from more knowledgeable people.

But Filipino-Mexicans are a historically well integrated community instead.

Semitic immigration was small in Mexico (unlike in South America) but a Lebanese-Syrian community is reported in Puebla.

Re: dark-skinned people, again I sense prejudice, they are called the Spanish word for Buckwheat?

This got me intrigued. I searched the Spanish terms for this cereal and they are "alforfón", "trigo sarraceno" (saracene wheat) and "trigo negro" (black wheat). Which term is specifically used in Mazatlán? Also by "dark-skinned people", do you mean Afro-Mexicans or just people with greater native american ancestry? Is the term used spitefully or just harmless slang?

Note: gringos are tolerated, but generally aren't encouraged/permitted to become mexican citizens - that is apparently quite hard to do?

I don't know about Mexican citizenship laws but certainly US citizens are the biggest immigrant community in Mexico and the largest community of expatriate US Americans in the world as well.

The former President, Vicente Fox, is the son of a US immigrant from Ohio.

re: segregation. Like many societies it can be subtle - but its there and it usually is social class (caste) - read that as money. The mestizo class is well adjusted and is the largest. They are building up a middle class (home ownership, auto etc., english as a second language).

You mention Fox, one other issue is simply height, the taller the better (Fox was over 6 feet). The native Indios are short 4'8 to 5'2 or so. Tall chicas are sought after.

I know that black-americans are treated uniformly in mexico, but I don't think that "sarassin" is considered attractive. (Note the first european settlers were French and German). Lizarraga is probably the most common name in Mazatlan.

You are right! Guadalajara has a large settlement of Canadians and US citizens. Just in my experience, very few obtain mexican citizenship. Part of that is land ownership. If you are not a mexican citizen ( or hold dual citizenship), you can't own land within 50 miles from the ocean.

None of my comments are "scientific", it is just the impression I have after living with them for various amounts of time for the last 25 years. We use quite a bit of Mexican labor, predominantly Oaxaca/Chiapas in my vineyard. Con mucho gusto Senor Maju.

Maju responded: "That's a blank statement and, sincerely, I doubt it to be true for most Mexicans."

Maju, I'm curious why you think that? I have no experience living in Mexico and so can't speak to the veracity of MCG's statements, but since MCG has lived there, I wouldn't be inclined to reject his/her observations out of hand. Of course, your comments could have been informed by personal experience in Mexico, in which case, your opinion is certainly valid and holds the same weight as MCG. You are usually a voice of reason on this blog, which is why I'm curious about your somewhat out of character response to MCG.

To bring the comments back around the the ancestry painting, I couldn't help but notice that in mixed-ancestry individuals, it still seemed as if there was a slight tendency for the chromosomal sections to line up across from other chromosomal sections of similar origen. (i.e., even though the chromosomes were of "mixed" origen, African genes showed a slight propensity to line up against other African genes, and European with European, etc.)

I have a couple of ideas why this would be. I'm curious what others think.

@Kosmo: I have never lived in Mexico but from direct and mostly indirect relation with Mexicans, reading Latin American sites in Spanish and so on... I have the strong impression that the sentence I quoted (... mexicans are quite in favor of "segregation"...) is surely not extensible to most Mexicans. I just don't think Mexicans are in favor of segregation, no matter that some have some racist tics.

One thing is to be grown up in a culture with traditional race castes and another thing being being in favor of keeping such cultural barriers or even incrementing them into a segregated regime such as Jim Crow or whatever.

Another element to consider is that Mexico, maybe even more than other Latin American countries, has its identity built on the very concept of mestizage, admixture. They typically consider Mexico not a multirracial society but a admixed, a mestizo one - at least in theory.

The traditional Hispanic (and I would say Latin in general, even Mediterranean in general) ethnic discourse of "race" considers it an extension of language and culture not a biological trait. This is, of course, contradictory with implicit traditional racism inherited from colonial times.

McG may have interpreted the latter in Anglosaxon cultural and historical terms. And anyhow we do not know how many Mexicans and of what social class and racial category did McG dealt with. Surely living with mostly white or quasi-white upper class mestizos is not the same as living among the poor in the barrios of Mexico City or with the peasants in the country or with the natives in the mountains of Chiapas. And I am sure that the less favored classes, who make the vast majority of Mexico are not politically racist (what does not mean that they may not have personal contradictions of racist nature born from culture, history and tradition).

I know that black-americans are treated uniformly in mexico, but I don't think that "sarassin" is considered attractive. (Note the first european settlers were French and German). Lizarraga is probably the most common name in Mazatlan.

Lizarraga is a Basque surname meaning "place of elm trees".

I really don't know how they use the term "sarraceno" (I'm assuming that's the word) in Mexico and I know that the small Afro-Mexican community often has issues of police racism. But people in Latin America are often less susceptible to race slang, as long as it doesn't have bad intention - at least as far as I can tell.

There is no black racial political activism as far as I can tell anywhere in Latin America and instead the strategy of the once discriminated black or mulatto population, where it exists, seems to be full assimilation. The racial issues anyhow more often than not go along the lines of the native-european divide, largely buffered anyhow by the numerically dominant mestizo group.

The place where greater racism, specifically anti-black racism, may exist is paradoxically in the mostly mulatto Dominican Republic. This is largely caused by reasons of national identification against Haití, that once controlled the Spanish-speaking country.

The racist discourse has also arisen recently in Bolivia for instance lead by the affluent and still powerful white minority. But is an anti-native discourse in a country historically ruled by whites (and some mestizos) but majoritarily native american, at the time when a reformist native president has been elected.

"To bring the comments back around the the ancestry painting, I couldn't help but notice that in mixed-ancestry individuals, it still seemed as if there was a slight tendency for the chromosomal sections to line up across from other chromosomal sections of similar origen. (i.e., even though the chromosomes were of "mixed" origen, African genes showed a slight propensity to line up against other African genes, and European with European, etc.)"

What's wrong with ethnic segregation? And why do all the dark skinned people have to live in Mexico-do they even want to?People say Mexican Indians are short and Spanish folks are tall yeah, well so what,it only matters a great deal if you are trying to mix yer blood with either of them.I thought most White folks didn't like Mexicans ,no matter what their height;like actress Penelope Cruz from Spain-I didn't think she was all that different or friendly,when she first came here,all White people said was that she wasn't like them and she's rather tall.

re: segregation. The native indio is segregated by choice. He lives in the Sierras in villages on land owned by the gov't and is allowed to use it unless resources are found. As you say the mestizo really isn't bigoted, not that I can observe. If there is differentiation its along natural lines like; color, height and wealth. The upper class is highly segregated! A classic case of haves and have nots. The government is run by haves and the lawyer/business community as you might expect. The minimum wage is brutal. About $7.50 per day., the tourist trade where tips are given is the path "up". Learning English is of high priority to the Mestizos, it gives them a better job.

So, who lives the best?? Again, it is partly lifestyle. The Indios appear to have the most freedom and intermarriage appears rare now. The mestizos are homegenizing, getting taller and developing the same light brown skin. The "elite" remain that way and most intermarriage is either out of Mexican society(met someone at college in the US,e.g.) or within the close knit group they were raised in.

Summary: gene diversification is still going on with the mestizos, the indios are once again a remote society( no schools, no govt benefits, etc., very little intermarriage); the elite marry within their strata. The upper and lower "castes" are almost status quo; the mestizo is the growth segment of mexican society; in Mazatlan at least.

I sure hope the Nazi comment doesn't apply to me? I am just the messenger!!! Certainly, the separation between the Indio and the other two classes is now by choice. The "segregation" between the mestizo and the "elite" is much more subtle and is based on wealth, reputation, position in society, etc..

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